This invention relates to an electromagnetic switch device, and more particularly to improvements in an electromagnetic switch device for use with a starting motor device for engines.
An electromagnetic switch device for use with a starting motor device is conventionally generally known which comprises: a switch housing mounted on a rear housing of the starting motor device and extending rearwardly therefrom; a solenoid assembly accommodated within the switch housing and including a solenoid, a bobbin carrying the solenoid, and a yoke plate and an end plate arranged at the opposite ends of the bobbin; a movable core axially movably inserted through the solenoid assembly via a spring and carrying a moving contact at its rear end; and a cap fixed to the rear end of the switch housing and carrying a pair of fixed contacts disposed in face-to-face relation to the moving contact. The movable core is moved against the force of the spring upon energization of the solenoid to bring the moving contact into contact with the fixed contacts.
Problems in the conventional electromagnetic switch device constructed in this manner are poor smoothness of movement of the movable core and inaccurate operating position of the same.
For instance, the above-mentioned cap is joined to the switch housing in an airtight manner through a packing made of rubber for prevention of entry of alien substances into the cap. The airtightness of the internal space of the cap brings about the disadvantage that as the movable core is moved toward the stationary core, the pressure in the internal space increases above atmospheric pressure, which impedes smooth movement of the movable core. Further, the above arrangement can cause a secondary disadvantage, that is, if the air in the internal space of the cap increases in pressure by heat emitted from the engine, and then is suddenly cooled due to splash of muddy water or the like on the switch housing, the air pressure first increases and then reduces correspondingly so that external water is sucked into the internal space mainly through the junction at which the packing is located, and fouls the switch contacts in the form of droplets, causing damage to the same.
Another factor which impedes smooth movement of the movable core lies in the sleeve which is fitted in the bobbin along its inner peripheral surface. More specifically, a conventional sleeve is disposed along the substantially whole length of the yoke plate and the bobbin. To this end, the sleeve has a long surface for sliding contact with the movable core, which also impedes smooth sliding movement of the latter on the sleeve, requiring very close machine tolerances to ensure smooth movement of the movable core. Further, the sleeve has its one end secured to the yoke plate by means of an adhesive. The adhesive can flow out of the gap between the sleeve and the yoke member, impeding smooth sliding movement of the movable core.
The smooth movement of the movable core can also be impeded by distortion of the switch housing in which the coil assembly in accommodated. Distortion of the switch housing can be caused by a conventional joining method that only part of the rear end peripheral edge of the switch housing is inwardly bent or caulked to the cap in a manner preventing relative rotation.
On the other hand, according to the conventional arrangement, the starting position of the movable core is determined by the position where the tip of a hook protruding from the front end of the movable core comes into pressure contact with an associated wall surface of the rear housing of the starting motor device. However, the end face area of the hook must be small because of its limited mounting space, which causes the disadvantage that the small end face area of the hook strikes against the associated wall surface of the rear housing to form a depression in the same wall surface, resulting in a change in the starting position of the movable core.